1. Technical Field
The disclosure relates to an overload protection device, and more particularly, to an overload protection device for motors.
2. Description of Related Art
Electric motor as a piece of very common equipment is used for converting electrical energy into mechanical energy. Some times, the electric motor is easily damaged by over-current or over-heat during overload conditions. Therefore, varieties of motor overload protection devices are employed to prevent damages to the electric motors during the overload conditions. However, motor overload protection devices proposed heretofore suffer from various significant disadvantages. For example, motor overload protection devices have been proposed to be controlled by a microprocessor. Although the microprocessor provides a greater flexibility in the control features of the overload protection device by monitoring the thermal characteristics of the motor, such microprocessor must be electrically powered up, even when power is disconnected from the motor, in order to calculate the temperature of the motor. Thus, this type of continuously powered motor overload protection devices is vulnerable to any temporary power interruption. Without power supply, the microprocessor is simply unable to track any information indicative of motor cooling during unpowered condition. This is particularly undesirable since the lack of such information essentially disables or prevents the microprocessor from accurately resuming computation of motor temperature once power is resumed. Although schemes which employ battery backup and/or power backup capacitors may enable the microprocessor to track or compute the thermal state of he motor based on its thermal characteristics when the main power to the microprocessor is interrupted, these schemes generally results in devices that are relatively expensive, bulky, complicated and generally require more software support due to their inherent complexity.
Therefore, what is needed is to provide an overload protection device for a motor which is simple in structure and can provide accurate and stable protection for the motor.